A phased array antenna includes an array of antenna elements adapted to produce a plurality of collimated and differently directed beams of radio frequency (RF) energy. Beams are formed by weighing the amplitude and shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction.
Each element in the phased array, or a group of elements, is driven by a transmit/receive (T/R) module which includes a controllable attenuator (or amplifier) and a controllable phase-shifter adapted for controlling the amplification and phase shift of the T/R modules.
In order to obtain the desired directional properties of an antenna, it is necessary to minimize the side lobe levels of the antenna. To enable low side lobe levels with an electrically controlled phased array antenna, high accuracy of amplification and phase shift in the modules is required. In practice, this is achieved by calibration function of the phased array. The calibration concept is based on the compensation of amplitude and phase errors owing to the various contributions of cables, attenuators, phase shifters, regulators and other passive and active parts in the transmit/receive channels, which respond differently at different temperatures, for each antenna element and at each radio frequency. The calibration procedure is required to determine what controls should be applied to the T/R modules in order to obtain the desired current distribution on the antenna aperture.
For example, if it is required that the phase of the signal fed to all antenna elements be identical, but it is found during calibration that, owing to mismatches in the phase shifters coupled to the first and second antenna elements, there is a certain phase difference Δφ between the signals output by a first antenna element and a second antenna element, then the phase shift signal that is fed to the second antenna element must have a phase offset of −Δφ relative to the phase shift signal fed to the first antenna in order to compensate for the mismatch in the two phase shifters. Differences between the amplitudes of signals that are output by different antenna elements caused by mismatches in the gains of the amplifiers coupled to the antenna elements are compensated for in a similar manner by applying different gain offsets to the antenna elements relative to a given reference antenna element.
A phased array antenna must be calibrated during manufacture in the factory before being deployed in the field in order to ensure that the radiation pattern of the antenna meets antenna performance specifications. Calibration is performed in the far field or on the near-field antenna ranges.
During receiver calibration mode, for example, a sampling probe of RF energy is positioned in the near-field of the phased array antenna elements. The T/R modules are sequentially activated. The amplitude and phase of each radiating element is accurately measured through each T/R module amplitude and phase state. These data are used to develop correction coefficients that minimize the element-to-element random errors. The desired radiation pattern is then achieved by adjusting the T/R module amplitudes and phases as indicated by the correction factors.
During use, performance of the antennas may deteriorate over time since one or more antenna elements may get out of calibration. In addition, failed T/R modules must be replaced. Replacement of a failed T/R module during antenna maintenance is usually a routine procedure, however, since the replaced T/R modules inevitably have slightly different properties to the original antenna elements, the original offsets will not compensate for slight differences in the phase and gain characteristics of the phase shifters and amplifiers used to feed steering signals to the corresponding antenna elements. As a result, the T/R modules must be re-calibrated to correct the drift of the component characteristics or the module replacement.
The complete phase antenna array can, for example, be returned to the manufacturer for re-calibration in order to establish the new offsets. It is also known to perform the re-calibration procedure in the field. For this purpose, the phased array antenna usually includes a calibration network for which the required offsets are known for each phase shifter and amplifier. The purpose of the calibration network is to provide injection of a predetermined calibration signal to each antenna element and to the corresponding T/R module connected to the antenna element.
During the re-calibration procedure, a calibration signal is injected into the calibration network. The amplifications and phase shifts of the T/R modules are obtained by considering the change in amplitudes and phases of the calibration signal when it passes the replaced T/R module. The control signals controlling the attenuators and the phase shifters in the T/R modules can then be corrected so that the amplification and the phase-shift are made to coincide with the desired amplification and phase-shift.
Several calibration network systems are known which can be used to calibrate a phased array antenna. Thus a technique is known whereby a transmission line, such as a micro-strip line, is placed close to the antenna elements, thus creating a non-directional coupling between the antenna elements and the corresponding portions of the transmission line. The term “non directional coupling” refers herein to a case where the amount of energy transferred from a calibration network toward an RF front is approximately equal to the amount of radiating energy transferred from the calibration network toward the antenna elements for radiation. It should be noted that a calibration network of this type is highly sensitive to the presence of external objects, due to reflection of the radiated energy by these objects.
A calibration network is also known that includes a set of RF directional couplers (one coupler per antenna element) placed physically close to each of the antennas. The RF directional couplers are interconnected and driven by a passive network having a corporate feed point. The passive network splits the drive calibration signal in a predetermined manner so that the signal fed to each antenna element is known in advance and the phase and gain offsets are known and predetermined.
In use of such a calibration network, precise adjustment is required to ensure that the signals fed via the couplers to the antenna element are identical in amplitude and phase. Therefore, if parameters of the components of the calibration network change for any reason, e.g. owing to changes in ambient temperature that may induce changes to the lengths of connectors, such changes must be compensated for.
This requires complex and costly circuitry associated with the calibration network in order to combine the signals passing from the antenna elements to a single corporate feed connector.